1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly to a photoreceptor cleanerless image forming apparatus which overlappingly forms yellow, magenta, cyan, and black toner images and continuously prints color images.
2. Description of the Related Art
The technology indicative of this type of image forming apparatus is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 5-341643, for example. While this example shows the photoreceptor cleanerless image forming apparatus handling a single color, it is developed to a 4-drum tandem image forming apparatus for continuously printing color images. FIG. 5 is a schematic diagram exemplifying a 4-drum tandem image forming apparatus according to the conventional photoreceptor cleanerless system. An image forming apparatus 400 is used for electrophotographic copiers and printers. There are arranged four photoreceptor cleanerless image forming units 400a, 400b, 400c, and 400d in tandem (4-drum tandem system). The image forming units 400a, 400b, 400c, and 400d having the same configuration form and transfer yellow, magenta, cyan, and black images.
The image forming unit 400a comprises a photoreceptor drum 403a, a charger 405a (e.g., scorotron charger), an exposure apparatus 406a, a developing apparatus 409a (e.g., 2-component developing apparatus), a transfer roller 423a, a DC power supply 427a, a destaticizer 421a, and a brush roller 422a. The other image forming units 400b, 400c, and 400d comprise the same constituent parts. An aligning roller 414 feeds paper P at a specified timing. The paper P is transported on an endless transport belt 111 between the photoreceptor drum (also abbreviated to the photoreceptor) and the transfer roller. The transport belt 111 is hung between a driving roller 428 and a driven roller 429. When the paper passes through between the photoreceptor drum and the transfer roller, a toner image is transferred to the paper P from the photoreceptor drum due to a transfer electric field between the photoreceptor drum and the transfer roller. After each color has been transferred, the toner image formed on the paper is fixed by a fixing apparatus (not shown) arranged downstream.
No photoreceptor cleaner is provided when each image forming unit is configured according to the photoreceptor cleanerless system as mentioned above. The toner is not completely transferred to the paper P and partially remains as untransferred toner on the photoreceptor drum. After passing through the destaticizer, the untransferred toner is charged together with the photoreceptor surface by the charger (e.g., scorotron charger) and then is exposed. After passing through the charger, however, an electric potential of the untransferred toner is higher than a developing bias of the 2-component developing apparatus. When the development is performed, the untransferred toner is also collected to the developing apparatus. The photoreceptor cleanerless system is characterized in that the untransferred toner is collected if no cleaner is provided. It should be noted that a brush or a brush roller may be provided immediately before the charger.
During the transfer process as mentioned above, the toner on the photoreceptor is transferred to a transfer material (paper or intermediate transferrer) due to the transfer electric field. If the transfer electric field is large, the toner once transferred to the transfer material is again returned to the photoreceptor (reverse transfer phenomenon). The inventors consider the reverse transfer phenomenon as follows. The reverse transfer phenomenon frequently occurs when there is a large difference between the charged potential on the rear (normally equivalent to a ground potential) or surface of the photoreceptor and an actual value of the transfer bias. After the transfer material passes through a transfer nip, the charged amount for the toner on the transfer material increases compared to that for the toner on the transfer material before passing through transfer nip. On the other hand, the charged amount for the reverse transfer toner greatly decreases (positively charged). It is assumed that a Paschen discharge occurring near the transfer nip causes the reverse transfer phenomenon. It is important to solve how to suppress the reverse transfer that causes the transfer efficiency to decrease, toner particles to scatter, and the image quality to degrade. Since the photoreceptor cleanerless system particularly allows the developing apparatus to collect untransferred toner remaining on the photoreceptor, this system can decrease waste toner and prolong the photoreceptor life. However, there remains a problem of mixing toner colors in the developing apparatus if a plurality of colors of toner simultaneously causes the reverse transfer phenomenon.
It is possible to decrease the reverse transfer phenomenon by setting a low transfer bias when transferring the toner to the transfer material from the photoreceptor. However, setting a low transfer bias prevents the toner on the photoreceptor from being completely transferred to the transfer material, increasing the amount of untransferred toner. In the image forming apparatus based on the photoreceptor cleanerless system, untransferred toner or reverse transfer toner is not cleaned until passing through the development nip. For this reason, the untransferred toner or the reverse transfer toner is charged by the charger together with the photoreceptor surface during continuous printing, and then is exposed by an exposure source during a latent image formation process. Accordingly, these toners cause charged spots on the photoreceptor surface or an incorrect latent image formation. The incorrect latent image formation due to an exposure error is especially remarkable. There is a problem that a toner image reveals a decreased density or density spots in a solid image or a halftone image as an image hysteresis.